Many wireless communication protocols provide for transmitters, operating within a communication network, which are capable of transmitting at varying levels of RF output signal power. One reason for having varying levels of RF output signal power is to accommodate mobile transmitters, which may be located at a varying distance from a base station. In some instances, the wireless communication protocol requires that the signal being received by the base station is received at a relatively constant or fixed RF output signal power level.
Examples of such protocols include Code Division Multiple Access (CDMA) and Wideband Code Division Multiple Access (WCDMA). To accommodate this requirement, transmitter wireless devices such as wireless telephones, wireless personal data assistants (PDAs), pagers, two-way radios, and other types of wireless devices will transmit at one of several power output levels, dependent upon the level at which the signal is being received. Other examples, where the transmitted output power can be varied, include Enhanced Data Rates for Global Systems for Mobile Communications Evolution (EDGE) and Global System for Mobile communications (GSM) which provides for a range of output power control of mobile transmitters between 20 dB and 30 dB, which is controllable in steps of 2 dB, and earlier analog cellular standards, which call for seven 4 dB steps in power output levels of the radio transmitter. Further, multimode wireless devices are designed to transmit communication signals of different modulation schemes using a single power amplifier. Therefore, the single power amplifier must also be capable of transmitting at the power output levels required for each of the different modulation schemes.
The single power amplifier in the wireless device, however, is typically optimized for operation at the highest power level. Consequently, power amplifiers typically operate at less than optimal efficiency at low power levels, and therefore the power consumption of these amplifiers may be reduced if the operating efficiency is increased.
One previous technique, which has been used to enhance operating efficiencies at lower power output values, has included reducing the bias signal supplied to the power amplifier. A further technique, which has been used to enhance operating efficiencies at lower power output values, is to adjust the load impedance coupled to the output of the power amplifier. Adjusting the load impedance of the power amplifier allows the power amplifier to operate at a higher level of efficiency for different output power levels of the power amplifier. However, such techniques to enhance operating efficiencies may cause transmitter phase discontinuities in the RF output signal resulting in significant errors in a receiver when attempting to receive a signal transmitted with phase discontinuities. A significant phase shift, such as, for example, typically of 30 degrees to 90 degrees, occurs coincident with a load impedance change to the power amplifier. A phase shift less than 30 degrees or greater than 90 degrees may also occur.
Yet another technique for enhancing the efficiency of a power amplifier involves a power amplifier with a stage bypass or alternatively a power amplifier having the ability to turn off or adjust power supplied to individual stages within the power amplifier. However, these techniques also generate a phase discontinuity in the power amplifier. A significant consequence of phase discontinuity in the power amplifier is the degradation of transmitted data and an increased generation of spurious signals in the desired transmission channel and the generation of spurious signals entering adjacent channels in the communication system. For example, phase discontinuities cause corruption of the modulated signal resulting in a temporary degradation in the performance of receivers such as base stations listening to the affected channels. The frequent and repetitive use of the efficiency enhancement techniques may occur frequently enough to further degrade the communication channel.
According to one technique for reducing the effects of phase shifting, a phase shifter simply alters the phase of a signal into a power amplifier in response to a control signal from a microprocessor. However, this technique generally changes the phase of a signal without coordinating the timing of the phase compensation signal relative to the initiation of an efficiency enhancement technique. As a result, the phase compensation signal may actually increase the amount of phase discontinuity in the power amplifier because the phase compensation signal may be timed inappropriately with respect to the application of the efficiency enhancement technique. For example, if the phase compensation signal is not properly timed with respect to the initiation of an efficiency enhancement technique, the signals may not cancel each other but may result in a greater phase discontinuity in the power amplifier. Therefore, the phase change caused by the implementation of a phase compensation signal that is out of synchronization with the phase change induced by the initiation of an efficiency enhancement technique may not result in the reduction or elimination of the phase change induced by the efficiency enhancement technique. Depending on the relative timing of the signals, larger phase discontinuities in the power amplifier may occur due to the introduction of an unsynchronized phase compensation signal. As a result, this phase compensation technique may not improve but instead degrade the performance characteristics in the communication channel.